ADAPTATIONS OF BIRDS 61 



air, in a calm rapidity, in a breeze slowness, of succes- 

 sional stroke will characterise the beating of the wing. 

 Further, since the resistance of the air increases with 

 great rapidity as the speed freshens, the wing after 

 one downstroke has to be lifted home edgewise for the 

 next, so as not to stop the momentum ; and this 

 turning movement brings the wing not full-sail up to 

 the vertical, but slews the front edge, somewhat flexed, 

 to the front, and then extends it high above the head 

 ready for another powerful sweep. 



To gain support from the air and to assist flight, 

 the details of shape and feathering of the wing are 

 modified. The lower surface grips the air chiefly by 

 its front edge and tip, and as a jib sail, though narrow 

 and triangular, must be strengthened on its front edge, 

 so good fliers need strong edge feathers and narrow, 

 pointed wings. The feathers are elaborately airtight to 

 enhance that grip, yet are disposed so as to allow air to 

 flow edgewise between them and over the convexity of 

 the upper surface as it comes forward. The difference 

 in the width of the vane on either side of the feather's 

 stem is not without its meaning, for, when a flat body 

 is falling obliquely through the air, the pressure be- 

 comes greatest not at the centre but towards the front 

 edge, which tends to rise. If, therefore, the front webs 

 of the first eight flight feathers were equal in width 

 to the hind vanes, the outstretched wings would tilt 

 up and the bird's balance would be destroyed. It is 

 to avoid this disturbance that the outer webbing of 

 the feathers, upon which the main stress falls, is reduced 



